The subscriber numbers of mobile phone networks increase strongly every year; consequently, the capacity of analogue mobile phones is coming to an end in several countries in the course of the next few years. In addition, the analogue networks are frequently national. In order to solve the problems related to capacity and internationality, a general European GSM mobile phone network is being implemented. The GSM network is the first fully digital mobile phone network. Digital mobile phone networks are being established also in the US.
Installing new digital networks takes several years. Starting of a GSM network will begin in capitals and will gradually spread to form a network which covers the whole Europe. In the transition period, an existing analogue mobile phone network operating in the same frequency range can be used in parallel with the new digital network. Analogue networks operating in Europe in the same frequency range include NMT-900 in the Nordic countries and TACS in Britain. In the USA, the analogue AMPS network operates in the same frequency range, and the coming digital CDMA network as well as the dual-mode network.
In the analogue network the mobile phone receives and transmits an analogue signal; respectively, in the digital network, a digital signal. Thus, mobile phones provided for an analogue network cannot as such be used in digital networks of the same frequency range, and vice versa. Transition into new digital networks is almost indispensable since the analogue networks are overloaded. On the other hand, the new network is not very large at the very beginning, so that a parallel use together with the existing analogue networks would be desirable.
The problem of two networks can be solved in that a so-called dual-mode phone is specified which is able to operate both as a digital and an analogue phone. The dual-mode phone selects automatically either of the modes of operation to conform to the base station environment. For instance, in the future US dual-mode system this kind of phone is a necessity because also in digital mode, the paging channel trafficing is carried out in the channels of the AMPS system. In the digital systems as such, the dual-mode phone facilitates the transition period.
The dual-mode phone is larger in size and in weight than the single-mode phone so that it is unnecessarily bulky to carry when the user is located clearly in the range of operation of either the analogue network or the digital network. In the city centres where the analogue network is crowded, it is convenient to use the digital network, and in suburban areas where digital networks have not been initially installed, the analogue network has to be used.